1. Field of the Invention
The present invention relates to a hollow rack for use as a steering rack in a traveling body having wheels, for example, automotive steering system and a method and an apparatus for reducing the diameter of an end portion of this rack. Further, the present invention relates to a method for manufacturing the hollow rack for use in a rack and pinion used in an automotive power steering system or the like and a hollow rack, particularly to increasing the strength of an end portion including a screw portion.
2. Description of the Related Art
Conventionally, there has been known a method and an apparatus for forming a rack corresponding to a tooth profile of an upper die in a pipe material with a core metal inserted into the pipe material held by a die assembly including an upper die and a lower die (21st line on right column of page 1 to 29th line on left column of page 2 and FIGS. 1 to 4 of JP Patent Application KOKOKU Publication No. 62-16735).
A step portion for improving the assembly and disassembly properties around a tie-rod is formed at an end portion in the length direction of the hollow rack manufactured according to this technology.
JP Patent Application KOKOKU Publication No. 62-16735 (21st line on right column of page 1 to 29th line on left column of page 2 and FIGS. 1 to 4) describes nothing about any specific method and apparatus for forming the step portion at the end portion of the hollow rack.
The step portion is formed by grinding the outer periphery of an end portion of the hollow rack by cutting or pressing a compression die into the outer periphery of the end portion of the hollow rack to reduce the diameter of the end portion. The latter method is more advantageous than the former method in that the strength of the end portion of the hollow rack is not lowered.
By the way, the length between the stepped portion and the rack is determined depending on the steering system of various vehicles. To satisfy such a demand, the inventor of the present invention has implemented a processing for reducing the diameter of an end portion on one side continuous to the other end side in which the rack is formed of the hollow rack currently in the following procedure.
First, with the center of the rack in the length direction regarded as a first processing criterion, an end side portion of the hollow rack is cut out at a position apart from that by a predetermined dimension so as to form an end face which serves as a second processing criterion. Next, a press-in depth for a compression die to the end portion of the other end side of the hollow rack is set with a predetermined distance apart from this end face. Next, the compression die is pressed onto the outer periphery of the end portion of the other end side of the hollow rack up to this press-in depth so as to reduce the diameter of the end portion of the other end side. In this case, a supporting member which is set in contact with the end face which serves as the second processing criterion receives the press-in load of pressing in the compression die.
To set up the press-in depth of the compression die for reducing the diameter of the end portion of the other end side of the hollow rack, the end face of one end side of the hollow rack positioned on an opposite side to the end portion of the other end side is used as a criterion. In such a processing method, the portion of the one end side of the hollow rack needs to be ground with the center of the rack in the length direction regarded as the first processing criterion, whereby cost for that is required.
On the other hand, a solid rack for use in a power steering system of a vehicle includes a tooth portion which is engaged with a pinion at its intermediate portion and a threaded portion for connecting with a ball joint is formed at an end portion thereof.
The solid rack is formed of a solid material composed of tempered material such as S45C. Although the S45C has hardness of Hv240-285, the shaft portion positioned at the intermediate portion has hardness of Hv600 or higher by heat treatment after the tooth portion is formed by cutting. The threaded portion positioned at the end portion maintains hardness of the material for a processing.
FIG. 29 is a diagram showing an example of a hollow rack developed recently in place of the solid rack. A hollow rack 200 includes a hollow shaft portion 201, and a tooth portion 202 which is engaged with a pinion (not shown) is formed at an intermediate portion of this shaft portion 201 while an internal threaded portion 204 for connecting with the ball joint is provided in an end portion 203 thereof. In the same Figure, reference number 210 denotes a ball joint, and reference number 211 denotes a socket for connecting the ball joint 210 with the end portion 203 via an external threaded portion 212.
In the meantime, technology for high-frequency heating the hollow rack and technology for changing the heat treatment condition partially have been generally known (see, for example, JP Patent Application KOKAI Publication Nos. 63-297524 and 6-264147).
The above-mentioned hollow rack has the following problems. That is, with increased size of vehicles in recent years, input load from tire to the rack and pinion has tended to increase. However, although the shaft portion 201 can be supplied with a higher hardness by heat treatment, the hardness of the end portion 203 cannot be intensified for thread cutting. For this reason, when strength test (arrow K in FIG. 29) of inputting a load from a ball joint 210 disposed on the side of tire is carried out, sometimes a screw can loosen due to shortage of the strength of the end portion 203 or the internal threaded portion or the ball joint 210 can slip out as shown in FIG. 30.